One aspect of the invention relates to a method for compensating for interfering variables in an optical sensor, in which an optical signal in the optical sensor is influenced at least as a function of a measured variable to be detected, a wavelength of the optical signal fed in and an interfering variable. In addition, one aspect of the invention relates to a device for compensating for the interfering variables in an optical sensor, the optical sensor through which an optical signal passes being at least sensitive with respect to a measured variable, a wavelength of the optical signal and an interfering variable.
Such a method and such a device are known, for example from U.S. Pat. No. 5,764,046 or WO 95/10046 A1. In these known methods and devices, a measured value for a magnetic field strength or an electric field strength is determined by an optical sensor designed as a Faraday or Pockels sensor. By a suitable sensor arrangement and evaluation, an electric current can also be detected by a Faraday sensor, and an electric voltage can be detected by a Pockels sensor. Both the Faraday effect and the Pockels effect are based on influencing, as a function of the measured variable, the state of polarization of the input optical signal fed into the optical sensor.
In both sensor types, the measuring sensitivity depends on the wavelength of the optical signal fed into the respective optical sensor. This wavelength dependence is also used, inter alia, to adjust the sensor sensitivity.
However, a change in the state of polarization is caused not only by the measured variables to be detected (for example magnetic or electric field) but also by certain environmental conditions, such as the ambient temperature or mechanical vibration of the optical sensor. In order to compensate for these undesired interfering variable influences, methods are described in U.S. Pat. No. 5,764,046 and WO 95/10046 A1 which substantially use suitable evaluation of the output optical signal received in the receiver.
In the case of this compensation method, the influence of the interfering variable is only extracted in retrospect within the context of signal evaluation. Here, it is entirely possible that no complete division can be achieved between the measured-variable-dependent and interfering-variable-dependent component in the output optical signal. As a result, errors occur in these compensation methods, and the measured variable may be determined only inaccurately.
In order to avoid the problem of the separation between measured and interfering variables, which is no longer possible in retrospect, a temperature compensation method for a Pockels sensor is described in the technical article by T. Mitsui et al. “Development of Fiber-Optic Voltage Sensors and Magnetic-Field Sensors” in IEEE Transactions on Power Delivery, vol. PWRD-2, No. 1, 1987, pages 87 to 93, in which method the compensation of the interfering temperature influence is carried out directly in the Pockels sensor itself. For this purpose, a specific construction is used for the Pockels sensor, in which the temperature dependence of the Pockels effect used for the measured variable detection is to the greatest extent just mutually cancelled by the temperature dependence of the optical activity. This means that the measured-variable-dependent influencing of the input optical signal taking place in the Pockels sensor described is to a large extent influenced independently of the interfering variable comprising ambient temperature. Retrospective separation in the measured-variable-dependent and interfering-variable-dependent component in the output optical signal is therefore not required. The particularly advantageous compensation method described in the technical article is, however, possible only for the specific application, that is to say given precisely this material selection and given precisely these geometric dimensions of the Pockels sensor. It is a specific solution and cannot be transferred to other optical sensors.
A potential object of the invention is, therefore, to specify a method and a device of the type designated at the beginning such that the best possible interfering variable compensation is possible, in particular also for different types of optical sensors.